Multiple bubble sextant or other altitude measuring device



Dec. 8,1942.

T. L. THUFQLOW MULTIPLE BUBBLE SEXTANT OR OTHER ALTITUDE MEASURING DEVICE Filed Ocfi. 15, 1941 1 AvvEwrO/Q THO/4453 L. TU/QLOW H a w v. 5

srras N g F E MULTKPLE SUBBLE SEXTANT QR GTHER AlL'iiTUDE MEASG DEVECE (Granted under the act of March 3, 1883, as

amended April St 1923; 376 6. G. 757) 6 Claims.

The invention described herein may be manufactured and used by or for Government for governmental purposes, without the payment to me of any royalty thereon.

This invention relates to improvements in optical instruments such as sextants, quadrants, octants and similar devices for measuring altitude and employing a bubble or other gravity con.- trolled means such as a pendulum as a horizon reference means.

Sextants and other instruments of a similar character have long employed levels of the bubble type to serve as a horizontal reference means in place of the natural horizon and in a few instruments pendulums have been similarly em.- pioyed, and are generally known in the art as gravity controlled artificial horizons. The use of gravity controlled artificial horizon on a sextant or similar instrument in navigating an aircraft, has, however, certain disadvantages which are due to accelerations and pitching of the aircraft, i. c., under such conditions, the artificial horizon such as the bubble or pendulum is affected by the motion ofthe aircraft and no longer defines a reference axis perpendicular to the true vertical. The effects of linear accelerations can in general be minimized by careful piloting but the effects of small oscillations in pitch and roll are beyond the control of the pilot though the oscillations in roll are usualy of a less troublesome nature, and if the gravity controlled artificial horizon is only lightly damped so as to respond almost instantly whenever the instrument is tipped out of the vertical, the image thereof will oscillate or dance so ts wing and also if the gravity controlled cans is heavily damped the sluggish response 1. reef will create a greater error in attempting to attain an equal shift of the image of the celestiel body on each side of the apparent mean rion of the image of the artificial horizon. The above acceleration effects are the same even if the sextant is of the type disclosed in United States Patent No. 1,392,263 in which the images the celestial body and artificial horizon move he same direction so that if in coincidence at int in the field of view they will remain "'dence except for the effects of acceleraapp icant has found. however, that if in o the single bubble ordinarily employed as horizon, two or more bubbles or other urolled means are employed movable parallel with the line of sight and havit'is very difficult to estimate the midpoint diaerent degrees of damping it is possible by simultaneously observing the images of the bubbles or other means, to determine the mean position of the oscillation thereof much more accurately than with a single bubble. ena is not based on any mathematical relationship between the rates of response of the bubbles or other reference means but appears to reside in the fact that if two of the bubbles are more highly damped than the sensitive bubble, the amplitude of oscillation of the damped bubbles will be much less and serve to confine the possible mean point of oscillation thereof to a smaller range which when compared with the oscillation of the sensitive bubble enables the determination of the midpoint of the oscillation of the latter and of the bubble system as a whole to be more accurately determined. The location of the mean point of oscillation of the bubble system is determined without any reference to whethe oranot the bubbles ever become aligned at the mid-point of the oscillation thereof and hence there need be no dennte mathematical relation between the frequency of oscillation or rate of response of the respective 'rbubbles. In an instrument of the character described the principle of the invention may be applied to pendulums if the same are em-' ployed for use as artificial horizons by constructing the pendulums to have different frequencies or different degrees of damping.

The principal object of the invention is the provision in an altitude measuring instrument such as a sextant or the like, of a gravity controlled artificial horizon means including a plurality of liquid level bubbles or pendulums each having a different frequency or period or rate of response to oscillatory acceleration forces, and the said plurality of bubbles or pendulums serving as a common horizontal reference means.

Other objects of the invention will appear by reference to the detailed description hereinafter given and to the appended drawing in which:

Fig. i is a side elevation view partly in section illustrating the invention as applied to a direct pointing type of sextant of a well known construction, and;

Fig. 2 is a top plan view of the device illustrated in Fig. 1.

Referring now to Figs; 1 and 2, the reference numeral 9, generally, indicates a conventional telescope adapted to be pointed directly at the celestial body or other object to be observed and is provided with an objective lens 2; spaced image erecting lenses 3, an inclined clear glass mirror l mounted between the lenses 3, and a conventional eyepiece or occular 5 located at the end of This phenom the focal length or" the optical system tr it t iii the telescope. The telescope i is provided with an aperture 6 through which the image of the artificial horizon is projected onto the mirror The telescope is rigidly secured to a plate memher l, to which is secured a handle 8, and an arcuate scale ll, the latter being secured at its upper end to the telescope i A pivot ill extending at right angles from the plate 'i serves to pivotally mount an arm i l at its inner end, the arm adjacent its outer end being provided with a vernier scale 22 which is adapted to cooperate with the indicia engraved on scale ll. The arm. ii at its outer end is provided with a'iriction roller iS-vvhich engages the periphery of the arcuate scale 9 and is rotated by a knurled wheel i lso as to rotate the arm it about its pivot to any desired position Within limits of ninety degrees of movement.

On the arm i l intermediate its ends mount-- ed an inclined silvered reflector 55 immediately over which is mounted a bubble assembly constituting a gravity controlled artificial horizon and generally indicated by the reference nu meral it and including three level bubble tubes ll, i8 and i9 respectively, and the liquids in the respective bubble tubes diiiering in viscosity so that the bubble in tube it is only slightly damped and is a so-called "fast bubble while the bubbles in level tubes i8 and iii are respectively increas ingly damped. A third bubble tube as is argo ranged transversely of the bubble tubes il, and i9 and is employed to indicate lateral tilt and is per so well known in the art. For taking sights at night a lamp (not shown) for illuinihating the bubbles from the side may be pro vided.

The images of the bubbles i l to is inclusive are reflected by mirror l5 through a collimating lens 22 onto a reflecting glass prism 23 which reflects the images of the bubbles onto the clear glass re" fleeting mirror i of the telescope l, the then being visible through the eyepiece The prism 23 is mounted on a support 25 which is provided with a bent over portion 25 which is formed integral with the outer end of a lever 25$ 45 which is pivotally mounted on the pivot iii behind the arm H. The lever 28 is provided with a radial slot 2? into which is fitted a guide pin or crosshead 28 which is rigidly secured to the outer end of a linli: as and serving as'a pivot for the outer end of a similar link 35, The links 29 and are respectively pivotally connected at their outer ends to the plate member 3 and the i l and a slot Si is provided in the member 5 to provide for arcuate movement for the bent over portion of the prism support 2 As the aim i 5 is rotated relative to the plate member I, the pin 28 and links 29 and 38 act in a manner equivalent to a compass straightening device so that the centerline of lever 25 always bisects the th cludedangle between arm ii and plate member 5, and prism 23 is thus rotated through one-hali of the angle of rotation of arm it relative to scale Q.

In the sextant as illustrated in the the radius of curvature of the bubble tu as mitting the bubble images is made equal to equivalent focal length of the lens system telescope, so that except for the effects or a celerations, the images of the celestial the artificial horizon reference means move the same direction an equal amount and when brought in coincidence will thereafter remain in coincidence.

With this construction the response of the bubbles is controlled by using liquids having the requisite viscosity in the respective level tubes, but it is to be understood that the bubble characteristics may be similarly varied by altering the radius of curvature of the bubble tubes in that class of instruments in which the images must be made to coincide with a reference such as a cross hair placed in the field of view and in this class of instrument the bubble characteristics may be controlled by selection of the radius of curvature of the level tube or of the viscosity of the liquid employed in the level tubes, or by a combination of both.

Operation In using the sextant in accordance with the invention the telescope l is pointed directly toward the celestial body to be observed and the arm H rotated by operation of thumb-Wheel M until the image of the celestial body and the imaginary transverse line passing through the bubble images coincide, the elevation angle then being read on scale t. if, however, the sextant is being shifted out of the vertical by acceleration forces such as arise due to pitching of an aircraft, the image of the celestial body will appear to oscillate and the images of the bubbles in level tubes ll, 98 and i9 will also appear to cs cillate, each with a different ampliimde and out of phase in varying degrees with the moving image of the celestial body. Since it is impossible under the above conditions due to lag to bring the image of the celestial body into coincidence even with the image of the fast bub ble ll, so that it becomes necessary to estimate the means position of the oscillation of the bub ble system which is rendered more accurate by comparison of the motion of the three bubbles ll, it and It as noted above, and the arm ii is adjusted until the image of the celestial body appears oscillate equally on either side of the apparent mean oscillation point of the bubble system the angular position. of the arm M relative to the scale a is taken as a measure of the altitude of the celestial body. The bubble in level tube Ell is employed a well known manner to indicate lateral tilt of the sextant. A number of separate altitude determinations are made and averaged to obtain the actual altitude of the celestial body'and it has been found that the average reading obtained with a sextant employing a multiple bubble artificial horizontal is considerably more accurate under conditions of acceleration than readings on an instrument employing only a single bubble.

It is apparent that the principle of the invention may be applied to altitude measuring instrirnents employing pendulums or equivalent gravity controlled means and the invention is applicable to any sextant to replace the single bubble or other gravity controlled artificial horizon.

Though one form of the invention has been illustrated and described, various modifications thereof will be apparent to those skilled in the art as falling within the scope of the invention as defined by the appended claims.

I claim:

1. in an instrument for measuring the altitudeof a celestial or other body with respect to a horizontal reference plane, gravity controlled means for establishing said reference plane, and rotatable means for bringing the image of said body into coincidence with the image of said gravity controlled means, said gravity controlled means comprising a plurality of gravity controlled elements movable in parallel planes and each having a difierent rate of response to the tilting of said instrument out of the vertical.

2. The structure as claimed in claim 1, in which the gravity controlled means comprises a plurality of liquid level bubble tubes arranged in parallel, each of said bubbles having a different degree of damping and the images of which are simultaneously visible to serve as a horizon reference.

3. The structure as claimed in claim 1, in which the gravity controlled means comprises a plurality of liquid level bubble tubes arranged in parallel, the liquids in said bubble tubes respectively differing in viscosity. v

4. An artificial horizon attachment for sextants and the like comprising a plurality of liquid level bubble tubes arranged so that the respective bubbles move in planes parallel to the axis of sighting, each of said bubble tubes having the same radius of curvature and the liquids therein respectively difiering in viscosity.

5. An artificial horizon attachment for sextants and the like comprising a plurality of liquid level bubble tubes arranged so that the respective bubbles move in planes parallel to the axis of sighting, each of said bubble tubes being so constructed that the bubbles have a respectively 'level tubes arranged in parallel relation, each of said level tubes having the same radius of curvature'and the bubbles in said tubes being simultaneously visible, the liquids in said level tubes respectively differing in viscosity such that the rate of response of the bubbles in said tubes respectively differ from each other.

THOMAS L. THU'RLOW. 

